I would not attempt to supply supplemental heat as this will change the bees metabolism such that they will attempt to fly (into the cold outside) and either die or use us a ton of energy in the process (requiring more food). The bees will natural cluster to stay warm.

The biggest threat to bees during winter months is starvation. Make sure they have enough honey and pollen reserves in the fall to get them through to the spring flow. If you're not sure, feed sugar syrup. It's not ideal, but better than losing a hive to starvation. Replacing bees is expensive.